Maternal-fetal transmission is felt to be responsible for a large number of chronic carriers of HBsAg. The mechanism by which the virus is transmitted from mother to infant is however unclear with the three main possibilities being (1) transplacental transmission prior to birth, (2) swallowing of maternal blood at the time of delivery and (3) post partum transmission via breast feeding or maternal handling. We will attempt to sort these possibilities by delivering the infants of HBsAg-positive chimpanzees by Caesarian section and by having the infant reared separate from the HBsAg-positive mother.